The present invention relates to calendering of a fibrous web, such as a paper and board web.
Calendering is a method by means of which the properties, in particular the thickness profile, smoothness, gloss, surface porosity and translucence of a web-like material, such as a paper web, are sought to be generally improved. In calendering the paper web is passed into a nip which is formed between rolls pressed against each other and in which the paper web is deformed by the action of temperature, moisture and nip pressure, in which connection the physical properties of the paper web can be affected by controlling the above-mentioned parameters and the time of action. The good physical properties attained by calendering lead to better print quality, thereby bringing a competitive advantage to the manufacturer of paper.
Shoe and/or extended-nip calendering is calendering in a calender unit in which a nip is formed between a roll provided with a resilient shell, the shell of said roll being made, for example, of polyurethane, and a press roll or shoe roll which has a rigid shell and is provided with an internal loading shoe and which is made of metal, such as steel.
In a multi-roll calender, the paper web is calendered by passing the web through nips defined between calender rolls arranged one upon the other to form a stack of rolls. The rolls are mounted on the frame of the calender and they are arranged to be in nip contact with one another such that the paper web can be calendered by passing it through the nips and by arranging a given load between the rolls in order to provide a linear load in the nip. For example, Finnish patent 95062, which corresponds to U.S. Pat. No. 5,590,593, discloses a calender in which the two lowest rolls in a stack formed by four rolls are deflection-compensated rolls, advantageously zone-controlled rolls and they are mounted so as to be stationary, i.e. their bearing housings are rigidly suspended from supports mounted on the frame of the calender. The two topmost rolls, of which the highest one is also a deflection-compensated roll, preferably a zone-controlled roll, are mounted to be movable such that their bearing housings are attached to loading arms pivotally mounted by means of articulated joints on the frame of the calender. The highest deflection-compensated roll can be provided with a resilient coating and the second topmost roll is a heatable so-called thermo roll. The lowest deflection-compensated roll in the roll stack can also be provided with a resilient coating. The calender has alternative modes of operation, such as, for example, passing the paper web to be calendered through all three nips with the nips closed or, by opening the topmost nips, through only two or one nip, respectively. It is mentioned that the advantages include, among other things, the fact that the calender principally provides all the same advantages as are achieved by conventional soft calenders, in addition to which the middle nip in the calender is a hard nip, by means of which caliper can be equalized. Moreover, the modifiability of the calender makes it suitable for many paper grades.
Similar modifiable calenders are also disclosed in European application publication 890676 and in GB application publication 2,119,422. European application publication 890676 discloses a calender in which six rolls are attached to a frame to form a calender stack. The topmost roll is fixedly attached to the frame, the middle rolls are attached to loading arms, and the bottom roll is arranged in a carriage which can be displaced in a vertical direction in a guide. The topmost and lowermost rolls are deflection-compensated rolls, and the topmost and lowermost rolls as well as the two middle rolls are rolls provided with an elastic coating, while the second lowest roll and the second topmost roll are hard-surface rolls. It is typical of the calender that when the topmost and lowermost nips are closed, the other nips therebetween are open, so that the web can be calendered symmetrically by means of the topmost and the lowermost nip and conducted without calendering through the open nips and via the take-out rolls situated adjacent to the rolls which form the opened nips.
GB application publication 2,119,422 in turn relates to an arrangement in which two stacks of three rolls are placed adjacent to each other on the same calender frame, the middle roll in each of said stacks being a hard roll and fixedly mounted on the frame and the soft rolls situated on both sides of the middle roll being displaceable to an operating position into nip contact with the hard-surface roll and away from it. These rolls situated in both stacks above and beneath the hard roll are deflection-controlled rolls. One of the soft-surface rolls in each roll stack serves only as a spare roll, which can be employed when the soft-surface roll proper which is in nip contact, for instance, becomes damaged, and thus the calender operates in practice as a double nip calender formed by two roll pairs.
FIGS. 5D and 5E of U.S. Pat. No. 4,332,191 show a calender which comprises two stacks of three rolls placed one upon the other, in which the middle roll in the upper stack is a soft roll and all rolls in the lower stack are hard rolls. Moreover, the calender includes a stack of three rolls on the other side of the frame, the middle roll in the stack being soft. The web can be calendered by passing it via the stacks provided with a soft middle roll or, by lowering the bottom roll of the upper stack into contact with the lower stack, a stack of four hard rolls is provided, and the web is passed through three nips of said stack.
There is also known a soft calender under the name of OPTIGLOSS™, which comprises two separate sets of three rolls, in both of which the middle roll is a hard thermo roll and there is a soft-surface roll on both sides of it. This provides a calender having 6 rolls and 4 nips in all.
As described above, it is typical of all calender structures that nips are opened by means of loading arms or by displacing other support structures supporting the calender roll, such as a carriage supporting the roll in guides extending parallel to the roll stack. This is necessary in order that the nips may be opened when changing the mode of calendering, i.e. the number or location of the nips which are calendering, for example, when the paper grade to be produced is changed. Likewise, it is typical of previous arrangements, like EP890676, that the mode of calendering is changed by opening some of the nips and by passing the web through the remaining closed nips. This leads to the fact that many rolls have to be moved.
Shoe rolls are typically hydraulically deflection-compensated, zone-controlled rolls in which the shell is supported from a non-rotating central shaft of the roll by means of a hydrostatic loading arrangement, such as a row of loading shoes, which transfer the nip force acting on the shell rotating around the central shaft so as to be carried by the central shaft. The loading element is generally also divided into zones, in which connection the loading pressure can be regulated as required by profiling. The zoning in this kind of zone-controlled shoe roll may comprise individual elements of the loading arrangement, in which connection the number of zones in the roll and in the loading arrangement may exceed 60—as examples may be mentioned the shoe rolls marketed by the Metso Paper, Inc. under the trademarks SymCD™ and SymCDS™, or the zoning may comprise a group of individual elements of the loading arrangement, in which connection the roll and the loading arrangement normally comprise eight zones—as examples may be mentioned the shoe rolls marketed by Metso Paper, Inc. under the trademarks SymZ™, SymZS™, SymZL™, and SymZLC™.
Shoe and/or extended-nip calendering has generally been found to be good for producing low-gloss paper grades, i.e. having a Hunter gloss % below 40, by calendering. When higher gloss is required, the nip pressure is, however, not sufficient to provide gloss. With ever-increasing running speeds, calendering is becoming a bottleneck in the papermaking process, and satisfactory quality is not achieved by today's machine calender units. Some of the drawbacks of the present papermaking process are also that                the loss of bulk increases when gloss is improved, and        in order to provide sufficient quality, it is necessary to use webs with an abundance of coating and/or to use off-line calendering, in particular multi-nip calendering, the known application of which is supercalendering, and/or soft calendering.        
The direction in the calendering technology is to an ever increasing degree towards on-line solutions even when the intention is to manufacture higher quality printing paper grades, such as SC and glossy coated papers. FIG. 1 illustrates how different printing paper grades are calendered today and, in particular, it can be seen where the calendering technology is heading in the near future.                a. Machine calendering is calendering in a calender unit in which nips are formed between non-resilient smooth-surface metal rolls placed one upon the other. The width of the nip in a machine calender is typically very small and the nip load in the nips is relatively high.        b. Multi-roll calendering, the known applications of which include supercalendering and, for producing SC-A and LWC grades in particular, a multi-roll on-line or off-line calender which provides in off-line operation in practice the best result in terms of quality, is calendering in a calender unit in which nips are formed between a smooth-surface metal or cast iron roll and a roll coated with a resilient coating, such as a paper or polymer roll, in which connection a wide nip is formed. The resilient-surface roll conforms to the contours of the surface of paper and presses the opposite side of paper evenly against the smooth-surface metal roll. Because of the resilient-surface roll, the calendering time is also longer, wherefore in order to assure adequate capacity, the paper machine must be equipped today even with three multi-roll calenders. In a multi-roll calender there are today typically from one to two sets of rolls and 10 to 12 nips. For the purpose of treating both sides of the web, the supercalender comprises a so-called reversing nip in which there are two resilient-surface rolls placed against each other. The linear load in the multi-roll calender increases from the top nip to the bottom nip because of the gravity of the earth. By using relief of the rolls, this increase in load can be compensated for. This kind of multi-roll calender provided with the relieving of the rolls is the multi-roll calender marketed by Metso Paper, Inc. under the trademark OptiLoad™.        c. Soft calendering is calendering in a calender unit in which nips are formed in a manner similar to that of a multi-roll or supercalender between a smooth-surface metal roll and a roll covered with a resilient coating, in which connection a wide nip is formed. In a soft calender, each nip is formed between separate roll pairs, in which connection the nip load in individual nips can be adjusted in each individual nip, or between roll pairs in separate sets of rolls, in which connection the nip load can be adjusted in each individual set of rolls. The soft calender can be used as an on-line calender and it can replace the machine calenders of a paper machine. In order to treat both sides of the web in the calender, the order of the roll pairs or of the sets of rolls forming the successive nips is reverse with respect to the web such that the resilient-surface roll or the resilient-surface rolls can be caused to work on both surfaces of the web.        